RES Scenario in the European Union - The Sustainability of Renewable Energy in Europe - page 41

The energy possessed by the vapor, expanded in a turbine coupled to a genera-
tor, is first transformed into mechanical energy and then into electrical energy. The
exhausted steam is conveyed into the condenser, where it is converted to water
at high temperature, which passes into the cooling tower, where it is cooled and
injected underground. It is of fundamental importance to ensure the renewability
of the geothermal system, replacing the fluid removed from use, both to prevent
landslides and in order not to deplete the geothermal fields. This can occur naturally
through the rain, while in other cases it is necessary to artificially recharge through
injection wells.
The conversion of geothermal energy into electricity is obtainable with differ-
ent technologies depending on the temperature and pressure of the hydrothermal
system available:
• Dry-steam systems that consist in majority by dry steam which is located at high
pressures and temperatures accompanied by other gases or soluble substances.
The steam can be used directly for the production of electricity conveying it to a
turbine.
• Systems to wet steam: it is constituted by hot water at a temperature higher than
its boiling point and high pressure; when pressure is reduced in the column of the
well the water vaporizes and comes to the surface in the form of a mixture com-
posed of water and steam. The steam can be used for the production of electric-
ity, while the hot water can be used in desalination plants to produce freshwater.
The temperature in this type of system is between 180 and 370 °C.
• Hot-water systems: they contain water at a temperature below 100 °C can be
used in most cases for direct uses such as home heating, greenhouses, and indus-
trial plants.
The use of geothermal energy can be divided into some areas: as uses at high en-
thalpy with fluids that reach temperatures higher than 150 °C; it concerns the pro-
duction of electric energy and some industrial uses and uses at medium- and low-
enthalpy fluids that reach temperatures of 150-100 °C in the first case and less than
100 °C in the second; and it concerns the direct uses as domestic, agricultural, and
industrial ones.
The overall yield of the production of electricity from geothermal steam is 10-
17 %, about three times lower than that of traditional sources, this is due to the
lower temperature of the steam (250 °C), both for the different chemical composi-
tion of the same, which determines a loss of energy. Geothermal power stations
consume from 6 to 15 kg of steam and a good production well, with a flow rate of
70,000 kg/h, can feed a power of 10 MW. On land, there are large areas in which the
subsoil are fluids at temperatures between 40 and 100 °C, easily accessible, which
could be directly used for heating and cooling, thus allowing a considerable saving
of hydrocarbons.